Independent mobility, the ability of an individual to successfully navigate through their environment, is a primary contributor to quality of life. Unfortunately, mobility often decreases following a stroke, preventing many people with stroke from returning to typical levels of activity participation. A major contributor to reduced mobility is gait instability, which can limit function either by increasing the risk of falls or by decreasing an individual's confidence in their balance. Existing rehabilitation techniques have been shown to improve certain aspects of post-stroke gait function, such as increasing self-selected walking speed. However, these interventions have generally failed to address gait instability, as evidenced by a lack of improvement in fall risk. The long term objective of this line of research is to develop a toolbox of interventions for improving post-stroke gait instability, allowing clinicians to select the most appropriate option for individual patients. This project will focus on a potential mechanistic cause of instability, which has been suggested by preliminary work that identified a consistent neuromechanical gait stabilization strategy among controls. The present approach contrasts with typical investigations of gait instability, which largely focus on quantifying non-causal indicators of stability rather than understanding the underlying mechanism. The central hypothesis is that post-stroke gait instability is commonly caused by a lack of mechanically- appropriate adjustments in foot placement. Individuals with stable gait patterns consistently use a strategy of actively controlling their mediolateral foot placement based on the mechanical state of their center of mass. However, this evidence of active stabilization is lacking in a sub-population of stroke survivors with an increased fall risk. The proposed work will investigate the effects of a novel force-field designed to allow the real-time manipulation of mediolateral foot placement without interfering with gait progression. The first Specific Aim is to determine whether foot placement assistance promotes use of the typical gait stabilization strategy. The force-field will be used to encourage mechanically-appropriate foot placement while stroke survivors walk, which may drive restoration of gait stability by allowing participants to practice the desired movement pattern in an appropriately challenging environment. The second Specific Aim is to determine whether foot placement perturbation promotes use of the typical gait stabilization strategy. The force-field will be used to perturb foot placement away from mechanically-appropriate target locations, potentially causing error-driven adaptation toward the desired gait pattern as participants seek to cancel out these errors. The proposed project is based on a quantitative understanding of human gait mechanics, and takes advantage of a recently developed force-field designed to allow gait manipulations that were previously not feasible. The resultant knowledge has the potential to make an important contribution to the development of a larger-scale rehabilitation paradigm in which therapeutic interventions are targeted to a patient's specific limitations.